<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
body {
width: 100%;
height: 100vh;
background-color: #000;
background-image: radial-gradient(circle at top right, rgba(121, 68, 154, 0.13), transparent),
radial-gradient(circle at 20% 80%, rgba(41, 196, 255, 0.13), transparent)
}
canvas {
position: fixed;
width: 100%;
height: 100%;
}
a {
position: absolute;
bottom: 2vmin;
right: 2vmin;
color: rgba(255,255,255,0.2);
text-decoration: none;
}
a:hover {
color: #fff;
}
</style>
</head>
<body>
<canvas></canvas>
<script type="text/javascript">
const STAR_COLOR = '#fff';
const STAR_SIZE = 3;
const STAR_MIN_SCALE = 0.2;
const OVERFLOW_THRESHOLD = 50;
const STAR_COUNT = ( window.innerWidth + window.innerHeight ) / 8;
const canvas = document.querySelector( 'canvas' ),
context = canvas.getContext( '2d' );
let scale = 1, // device pixel ratio
width,
height;
let stars = [];
let pointerX,
pointerY;
let velocity = { x: 0, y: 0, tx: 0, ty: 0, z: 0.0005 };
let touchInput = false;
generate();
resize();
step();
window.onresize = resize;
canvas.onmousemove = onMouseMove;
canvas.ontouchmove = onTouchMove;
canvas.ontouchend = onMouseLeave;
document.onmouseleave = onMouseLeave;
function generate() {
for( let i = 0; i < STAR_COUNT; i++ ) {
stars.push({
x: 0,
y: 0,
z: STAR_MIN_SCALE + Math.random() * ( 1 - STAR_MIN_SCALE )
});
}
}
function placeStar( star ) {
star.x = Math.random() * width;
star.y = Math.random() * height;
}
function recycleStar( star ) {
let direction = 'z';
let vx = Math.abs( velocity.x ),
vy = Math.abs( velocity.y );
if( vx > 1 || vy > 1 ) {
let axis;
if( vx > vy ) {
axis = Math.random() < vx / ( vx + vy ) ? 'h' : 'v';
}
else {
axis = Math.random() < vy / ( vx + vy ) ? 'v' : 'h';
}
if( axis === 'h' ) {
direction = velocity.x > 0 ? 'l' : 'r';
}
else {
direction = velocity.y > 0 ? 't' : 'b';
}
}
star.z = STAR_MIN_SCALE + Math.random() * ( 1 - STAR_MIN_SCALE );
if( direction === 'z' ) {
star.z = 0.1;
star.x = Math.random() * width;
star.y = Math.random() * height;
}
else if( direction === 'l' ) {
star.x = -OVERFLOW_THRESHOLD;
star.y = height * Math.random();
}
else if( direction === 'r' ) {
star.x = width + OVERFLOW_THRESHOLD;
star.y = height * Math.random();
}
else if( direction === 't' ) {
star.x = width * Math.random();
star.y = -OVERFLOW_THRESHOLD;
}
else if( direction === 'b' ) {
star.x = width * Math.random();
star.y = height + OVERFLOW_THRESHOLD;
}
}
function resize() {
scale = window.devicePixelRatio || 1;
width = window.innerWidth * scale;
height = window.innerHeight * scale;
canvas.width = width;
canvas.height = height;
stars.forEach( placeStar );
}
function step() {
context.clearRect( 0, 0, width, height );
update();
render();
requestAnimationFrame( step );
}
function update() {
velocity.tx *= 0.96;
velocity.ty *= 0.96;
velocity.x += ( velocity.tx - velocity.x ) * 0.8;
velocity.y += ( velocity.ty - velocity.y ) * 0.8;
stars.forEach( ( star ) => {
star.x += velocity.x * star.z;
star.y += velocity.y * star.z;
star.x += ( star.x - width/2 ) * velocity.z * star.z;
star.y += ( star.y - height/2 ) * velocity.z * star.z;
star.z += velocity.z;
// recycle when out of bounds
if( star.x < -OVERFLOW_THRESHOLD || star.x > width + OVERFLOW_THRESHOLD || star.y < -OVERFLOW_THRESHOLD || star.y > height + OVERFLOW_THRESHOLD ) {
recycleStar( star );
}
} );
}
function render() {
stars.forEach( ( star ) => {
context.beginPath();
context.lineCap = 'round';
context.lineWidth = STAR_SIZE * star.z * scale;
context.globalAlpha = 0.5 + 0.5*Math.random();
context.strokeStyle = STAR_COLOR;
context.beginPath();
context.moveTo( star.x, star.y );
var tailX = velocity.x * 2,
tailY = velocity.y * 2;
// stroke() wont work on an invisible line
if( Math.abs( tailX ) < 0.1 ) tailX = 0.5;
if( Math.abs( tailY ) < 0.1 ) tailY = 0.5;
context.lineTo( star.x + tailX, star.y + tailY );
context.stroke();
} );
}
function movePointer( x, y ) {
if( typeof pointerX === 'number' && typeof pointerY === 'number' ) {
let ox = x - pointerX,
oy = y - pointerY;
velocity.tx = velocity.tx + ( ox / 8*scale ) * ( touchInput ? 1 : -1 );
velocity.ty = velocity.ty + ( oy / 8*scale ) * ( touchInput ? 1 : -1 );
}
pointerX = x;
pointerY = y;
}
function onMouseMove( event ) {
touchInput = false;
movePointer( event.clientX, event.clientY );
}
function onTouchMove( event ) {
touchInput = true;
movePointer( event.touches[0].clientX, event.touches[0].clientY, true );
event.preventDefault();
}
function onMouseLeave() {
pointerX = null;
pointerY = null;
}
</script>
</body>
</html>